A speed ratio and a transmission torque capacity of a vehicle transmission are customarily altered by a hydraulic pressure. For example, Japanese Patent Laid-Open No. 2010-151240 describes a hydraulic control device of a belt-driven continuously variable transmission. The hydraulic control device is comprised of a low pressure circuit to which low pressure fluid regulated by lowering an initial pressure of a pump is delivered, and a high pressure circuit to which high pressure fluid higher than said low pressure is delivered. The low pressure circuit includes friction sites of the continuously variable transmission and lubrication sites such as a bearing. The high pressure circuit includes hydraulic chambers of a drive pulley and a driven pulley to which a drive belt is applied, and an accumulator storing high pressure fluid. The drive pulley is provided with an intensifier valve for supplying the fluid to the hydraulic chamber, and a depressurization valve for draining the fluid from the hydraulic chamber. Also, the driven pulley is provided with an intensifier valve for supplying the fluid to the hydraulic chamber, and a depressurization valve for draining the fluid from the hydraulic chamber. A communication between the accumulator and the pump is provided through a check valve. According to the teachings of Japanese Patent Laid-Open No. 2010-151240, the hydraulic control device is configured to deliver the fluid from the accumulator to the chambers of pulleys to alter a speed ratio moderately. By contrast, the fluid is delivered from the pump to the chambers of pulleys to alter a speed ratio rapidly.
Japanese Patent Laid-Open No. 2009-97677 describes a control device for variable displacement pump motor type transmission. The control device is comprised of a pair of hydraulic pump motors, and a closed circuit formed by connecting an inlet side passage connecting inlet ports with an outlet side passage connecting outlet ports. A first relief valve is disposed on a communication passage connecting the inlet side passage and the outlet side passage. The first relief valve is opened when the fluid pressurized higher than a predetermined level leaks from the inlet port of one of the pump motors to discharge the highly pressurized fluid from the closed circuit. That is, the first relief valve maintains the pressure of the inlet side passage to the predetermined pressure. A second relief valve is disposed on another communication passage connecting the inlet side passage and the outlet side passage. The second relief valve is opened when the fluid pressurized higher than a predetermined level leaks from the outlet port of one of the pump motors to discharge the highly pressurized fluid from the closed circuit. That is, the second relief valve maintains the pressure of the outlet side passage to the predetermined pressure. Additionally, inlet side passage is connected with the accumulator though a check valve. According to the teachings of Japanese Patent Laid-Open No. 2009-97677, therefore, the highly pressurized fluid is accumulated in the accumulator by closing the first relief valve without discharging from the closed circuit when the pressure of the inlet side passage is raised.
As described, according to the hydraulic control device taught by Japanese Patent Laid-Open No. 2010-151240, the intensifier valve for drive pulley is opened when upshifting is carried out to reduce the speed ratio, so that the fluid is delivered from the accumulator or the pump to the chamber. Consequently, a groove width of the drive pulley is narrowed to increase a running radius of the belt. At the same time, the depressurized valve for the driven pulley is opened to widen a groove width of the driven pulley so that the fluid in the chamber of the driven pulley is discharged to a drain spot. Consequently, the hydraulic pressure in the chamber of the driven pulley is adjusted in accordance with the torque transmitting capacity thereof. Thus, according to the hydraulic control device taught by Japanese Patent Laid-Open No. 2010-151240, the highly pressurized fluid is discharged to the drain spot on the occasion of speed change operation. As a result of thus discharging the high pressure fluid during the speed change, an energy loss may be caused to worsen fuel efficiency of the vehicle.